Photolithographic masks (hereinafter referred to simply as photomasks) for use in the manufacture of semiconductor integrated circuit devices are largely broken down to two major categories, one including metal or hard masks and the other including emulsion type masks. Metal or hard photomasks use coatings of metallic chromium, chromium oxides, iron oxides, silicon oxides or any combinations of these on glass substrates. At least one coating of any of these materials is either deposited or sputtered onto the surface of the substrate. On the other hand, emulsion type photomasks are fabricated by application of emulsified silver halogenides to the surfaces of glass substrates. Of these two types of photomasks, the former, viz., metal photomasks are presently more widely accepted than the latter for their abilities to provide higher degrees of resolution and mechanical durability.
When such a metal photomask is put to repeated use, the mask is contaminated from various sources. These sources of contaminants include the air-born organic chemicals which tend to be deposited on the mask during storage of the mask, photoresist materials with which the mask is to be used during contact development processing, and fine particles which tend to stick to the mask during transportation and handling of the mask. While metal photomasks provide longer lifetimes than emulsion type photomasks for their enhanced mechanical durability, they thus require elaborate cleanups when used repeatedly over extended periods of time. Various techniques are operable for the cleaning of contaminated metal photomasks, including chemical rinsing processes using surfactants or organic or inorganic acids, scrubbing with use of sponges or brushes, and physical purging processes using supersonic waves. The chemicals which can be used for chemical rinsing purposes include nitric acid, sulfuric acid, a chromic acid mixture and an aqueous solution of hydrogen peroxide. While each of these cleaning techniques is useful to a greater or lesser extent when used independently or in combination with any of other techniques, none of them has proved fully acceptable for the cleaning of metal photomasks. Failure to completely clean photomasks inevitably leads to reduction in the yields achievable of the manufacture of semiconductor integrated circuit devices.
In the meantime, it is known that a solid object depends for its adaptability to cleaning on the surface state of the object or, more specifically, on the angle of contact between the surface of the object and a body of liquid which, in this instance, is typically pure water. The smaller the contact angle between the object and the body of pure water, the higher the efficiency at which the object can be cleaned with water. With this in mind, we have made extensive studies on the adaptability of photomasks to cleaning with pure water and have found that a photomask has an increased degree of adaptability to cleaning with water and is capable of maintaining such adaptability over a theoretically unlimited period of time when the metallic film of the mask is doped with ions of sulfur.
It is, accordingly, a prime object of the present invention to provide an improved photomask which has an increased degree of adaptability to cleaning with water over a practically unlimited period of time.
It is another important object of the present invention to provide an improved photomask which can be used repeatedly for the manufacture of semiconductor integrated circuit devices over an extended period of time.
It is still another important object of the present invention to provide an improved photomask which is adapted to achieve an enhanced yield and a reduced cost of production of semiconductor integrated circuit devices.
It is, yet, still another important object of the present invention to provide a method of fabricating such an improved photomask.